Track-type undercarriages, such as those individually having either a track chain or an elastomeric belt entrained about a rear driving wheel assembly and a front idler wheel assembly, are known to exhibit markedly superior performance characteristics in certain respects over wheel suspension systems. These track-type undercarriages provide reduced ground pressure per unit area of contact and increased traction. Moreover, uneven and soft terrain vehicles equipped with track-type undercarriage have improved flotation and stability, and are more efficient than wheeled vehicles.
Most crawler tractors have two track-type undercarriages which are pivotally mounted on aligned shafts to the rear of the main frame and which are adapted to limitedly swing in vertical, longitudinally oriented planes. Oscillation of the undercarriages is desirable to accommodate surface irregularities and to reduce shock loads, and the front portions thereof are often interconnected by an equalizer bar or generally equivalent leaf spring assembly which is centrally connected to the main frame. Exemplifying the art in this 3-point main frame suspension area are: U.S. Pat. Nos. 1,512,152 issued to R. H. White on Oct. 21, 1924; 2,786,724 issued to R. Q. Armington, et al on Mar. 26, 1957; 2,936,841 issued May 17, 1960 to J. Mazzarins; and 3,899,218 issued to G. D. Blomstrom, et al on Aug. 12, 1975.
The aforementioned patents are generally illustrative of the large number of coupling devices that have been proposed for connecting the equalizer bar to both undercarriages and to the main frame of the vehicle in order to accommodate the tendency of the front end of the undercarriages to pull inwardly towards each other during oscillation. In addition to accommodating such compound angular misalignment of the undercarriages, it is often desirable to avoid the complexities of providing a fixed power transfer mechanism for driving the rear wheel assembly. Historically, the undercarriages have been mounted on the main frame for pivotal movement about the transverse axis of the rear drive wheel assemblies. But, it has been found that when large implement forces are imposed on the bearings of the rear drive wheel assemblies, such as are encountered when using a bulldozer blade, there is a marked decrease in the surface life of the bearings and associated elements. Accordingly, the undercarriages have often been pivoted on shafts located typically forwardly of the axis of the rear drive wheel assemblies so that operating forces on the undercarriages and on the implement can be directed thereto independent of the rear drive wheel assemblies.
Because the axis of the drive wheel assembly is spaced away from the pivot axis, it travels in an arcuate path with oscillation of the undercarriage. This adds considerable cost and complexity to the mechanical drive train powering the rear drive wheel assembly.
It has been found that many combine harvesters equipped with wheels could not be operated under wet and muddy conditions at the time the crop was ready to be harvested. In order to overcome this significant agricultural problem, the CLAAS OHG combine harvester manufacturer of D-4834 Harsewinkel 1, Federal Republic of Germany has replaced each of the front drive wheels with a drive sprocket and has pivotally supported an undercarriage to the frame below each sprocket. The front and rear idler wheels on the undercarriage and the elevated drive sprocket are encircled by a multi-jointed, metallic track chain in a triangular configuration. This has enabled the combine to traverse safely over soft ground because the majority of the weight is more evenly distributed through the undercarriage and tracks. Steering of this combine, and other combines modified in substantially the same way, is still accomplished by the usual rear steering wheels which undesirably have relatively high pressure contact with the ground. Accordingly, this specialized attachment suspension system is partially track and partially wheels, so that the full benefits of a totally track-supported suspension structure are not realized. Moreover, such combines typically have a relatively light main frame utilizing thin beams, diagonal braces, and plates, and it is therefore difficult to connect the combine to separate extended-length undercarriages with the desired degree of structural integrity.
Therefore, what is desired is a suspension structure for a tracked vehicle such as a combine harvester that is rugged and economical, that can better distribute the weight of the superstructure to the ground, and that is compatible with the work implement on the vehicle during negotiation of uneven terrain. Moreover, the suspension structure should include laterally spaced apart undercarriages individually including front and rear wheel assemblies and an endless belt or track looped about the wheel assemblies, and wherein the undercarriages are individually pivoted for limited oscillation with respect to one end of the main frame for navigating and conforming to irregular and soft terrain. Means is also required for centrally supporting the other end of the main frame on the undercarriages while preventing uncontrolled lateral spreading thereof. Preferably, the suspension structure should not require any mechanical drive components to the drive wheel assemblies through the pivot shaft axes so as to avoid the complexities associated therewith, while also providing independent powered drive to both tracks for steering purposes. Also, for the greatest simplicity, the drive wheels should preferably be powered by individual motors releasably supported in a rugged manner to the undercarriage frames.